1. Field of the Invention
The present invention relates to improvement of a method of preparing a high-purity stable aqueous silica sol which has an SiO.sub.2 concentration of from 30 to 50% by weight and which is substantially free from any other polyvalent metal oxides than silica and in which the colloidal silica has a mean particle size of falling within the range of from 10 to 30 millimicrons.
In particular, the silica sol to be obtained by the method of the present invention has an extremely low content of oxides of polyvalent metals such as iron or aluminum and is therefore employed for the use which desires the absence of such metals in the silica sol, for example, as an abrasive for semiconductors.
2. Description of the Prior Art
A stable aqueous silica sol of industrial products which are used in various uses is prepared from water-soluble alkali metal silicates, in particular, from water glass of industrial products which are inexpensive and are easily available.
However, the stable aqueous silica sol to be prepared from water glass often has some problems because of the metal oxides which are generally in the silica sol in a slight amount, especially polyvalent metal oxides therein, in some uses, for example as an abrasive for polishing the surfaces of semiconductors, as a raw material for preparing quartz fibers or in any other uses which require a high-purity sol, since the techniques in the field have been high-leveled in these days.
A method of preparing an aqueous silica sol which is substantially free from alkali metal oxides is already known. For example, U.S. Pat. No. 4,054,536 illustrates a method of preparing an aqueous silica sol which is substantially free from alkali metal oxides, wherein an aqueous sol having pH of from 8 to 10 and SiO.sub.2 content of from 5 to 55%, which is obtained by adding an aqueous silicic acid solution with a concentration of from 2 to 10% to a hot aqueous solution of an alkanolamine in a proportion of from 1 to 100 as the molar ratio of SiO.sub.2 to the amine with removing water by distillation, is passed through ion-exchange resins . to thereby form an aqueous silica sol having an alkali metal content of 200 ppm or less and having pH of from 2 to 5, and thereafter ammonia is added to the resulting sol so as to make it to have pH of from 9 to 10 whereby a silica sol having an alkali metal content of 200 ppm or less is obtained.
Japanese Patent Application Laid-Open No. 61-158810 has proposed a method of obtaining a silica sol having a low content of any other polyvalent metal oxides than silica, wherein a silicic acid solution which is obtained by bringing an aqueous solution of an alkali silicate having a concentration of from 0.5 to 7% by weight into contact with a strong acid type cation-exchange resin for alkali-removal is treated with an acid at pH of 2.5 or less and at a temperature of from 0.degree. to 98.degree. C., then the impurities in the thus treated solution are removed by filtration through an ultrafilter, then the resulting solution is passed through a mixed bed composed of cation-exchange and anion-exchange resins and through a chelate resin layer, then ammonia or an amine is added to a part of the resulting oligo-silicic acid solution to give a heel solution having pH of from 7 to 10, and then the remaining part of the oligo-silicic acid solution is gradually and dropwise added to the heel solution at a temperature of 60.degree. to 98.degree. C. so as to grow the particles of the colloidal silica in the sol.
Where an aqueous silica sol is prepared by the method described in U.S. Pat. No. 4,054,536, using an aqueous solution of an active silicic acid to be obtained by passing a diluted aqueous solution of water glass of a commercial product, as a starting material, through a cation-exchange resin layer, an aqueous silica sol having an alkali metal content to silica of 200 ppm or less can be obtained. However, the aqueous silica sol to be obtained by the method is one having a content of other metals, for example, polyvalent metals such as iron or aluminum, to silica of being 300 ppm or more. The polyvalent metals which are in the aqueous silica sol in such a high content are derived from polyvalent metal oxides which are contained in the water glass of the starting material in a content of about 500 ppm or more.
In accordance with the other method as described in Japanese Patent Laid-Open Application No. 61-158810, all the alkali metals and other polyvalent metals than silicon which are in the alkali metal silicate of the starting material such as water glass are removed before preparing the silica sol. However, the removing step is not efficient as the process of industrial production of silica sol, since supplement of a separately prepared acid solution to the active silicic acid-containing aqueous solution is necessary in an amount corresponding to 20 to 10,000 times of the said aqueous solution in the step of removing the impurities from the aqueous active silicic acid solution containing the acid as previously added thereto by passing the aqueous solution through the ultrafilter. Additionally, in accordance with the method described in Japanese Patent Laid-Open Application No. 61-158810, a long period of 760 hours is necessary for the growth of the colloidal silica particles, and concentration of the resulting silica sol is effected after the completion of the growth of the particles. Therefore, such step of growing the silica particles to be followed by concentration of the resulting silica sol is not also efficient as the process of industrial production of silica sol.
Where a silica sol is employed as a binder, the sol may have a higher binding power when the particle size of the colloidal silica in the sol is smaller. On the contrary, however, the sol would be less stable when the particle size of the colloidal silica of the sol is smaller. Accordingly, in order to compensate the latter drawback, the SiO.sub.2 concentration in the sol is to be lowered. In general, however, when the sol is used as a binder, it is desired to have a sufficient binding power while it has a high SiO.sub.2 concentration. Regarding the particle size distribution of the colloidal silica in a sol, a broad size distribution would give a much higher binding power to the sol than those having a narrow size distribution.
Under the situation, it is one technical theme in this field to provide an improved silica sol that it may be helpful for enhancing the quality of the products to be obtained by the use of the silica sol, as inexpensive industrial products. The present invention is therefore to satisfy the theme and to overcome the problems in the above-mentioned conventional methods of producing a silica sol having a low content of other polyvalent metal oxides than silica therein. Specifically, the present invention is to provide a method of efficiently producing a stable aqueous silica sol, which has a content of other polyvalent metal oxides than silica being 300 ppm or less to silica, which has an SiO.sub.2 concentration of from 30 to 50% by weight and in which the colloidal silica has a mean particle size of falling within the range of from 10 to 30 millimicrons, from an alkali metal silicate containing polyvalent metal oxides as impurities.